Aluminum-lithium alloys, which exhibit reduced density and increased modulus characteristics, offer the potential of both weight savings and increased stiffness over conventional aluminum alloys used for aircraft components. Despite these potential benefits, the use of aluminum-lithium alloys in aircraft structural applications has been limited by their poor ductilities. The low ductilities exhibited by aluminum-lithium alloys have been attributed to the nonhomogeneous precipitation of .delta.' (Al.sub.3 Li), an ordered, shearable precipitate which promotes predominantly planar slip and intergranular fracture in these alloys.
In the peak aged condition, most age hardened aluminum alloys have a precipitate free zone (PFZ) along the grain boundaries. The PFZ is softer than the surrounding matrix and accordingly, is more easily deformed than the matrix in the age hardened condition. As a result, local deformation in the PFZ can be severe enough to initiate a crack at a grain boundary or at a grain boundary triple point before any substantial macroscopic deformation occurs. Once a crack has initiated, it may easily propagate along the grain boundaries. This mechanism leads to a microstructure with a low macroscopic ductility; however, other microstructural factors such as grain size, secondary dendrite arm spacing (SDAS), and elemental segregation may also influence the fracture characteristics of these alloys.
Due to the low ductility associated with coarse grain, cast microstructures, recent developmental efforts concerning aluminum-lithium alloys have been directed toward the production of homogenous, fine grain, wrought microstructures via rolling or forging of IM or PM billets. These activities have led to the introduction of several commercial wrought alloys including: Al-2.2 w/o Li-2.7 w/o Cu-0.12 w/o Zr; Al-2.2 w/o Li-1.1 w/o Cu-0.7 w/o Mg-0.8 w/o Zr; Al-1.7 w/o Li-1.8 w/o Cu-1.1 w/o Mg-0.04 w/o Zr; Al-1.9 w/o Li-2.5 w/o Cu-0.2 w/o Mg-0.04 w/o Zr; Al-2.3 w/o Li-1.25 w/o Cu-0.89 w/o Mg-0.13 w/o Zr; and Al-2.4 w/o Li-1.6 w/o Cu-0.5 w/o Mg-0.16 w/o Zr. One of the key features of these wrought alloys is the addition of zirconium to limit grain growth during thermomechanical processing which yields superior tensile strength and ductility properties. Additionally, strain age processing has been used successfully to promote secondary precipitation in PFZ regions and further enhance mechanical properties. Attempts to cast these wrought alloys using conventional investment casting techniques, however, have revealed that even relatively small amounts of zirconium tend to segregate undesirably which embrittles the castings. Thus, in spite of the excellent properties exhibited by these wrought alloys, they suffer from the disadvantage that they cannot be formed into net shape configurations. Consequently, in complex applications wrought materials are costly to produce because they require extensive machining. Thus, it is apparent that a method of forming net shape components from aluminum-lithium alloys would be desirable.
Accordingly, it is an object of the invention to provide a method of investment casting aluminum-lithium alloys to produce net shape components exhibiting the low density, high modulus characteristics of wrought materials, as well as suitable strength and ductility properties.
Another object of the invention is to provide an aluminum-lithium alloy composition for investment casting which exhibits a combination of strength, ductility, and density properties comparable to those of conventional cast aluminum alloys such as A356 and A357.
Additional objects and advantages will be set forth in part in the description which follows, and in part, will be obvious from the description, or maybe learned by practice of the invention.